Owing to its heat resistance, light weight, and superior weatherability, silicone rubber sponge is used for packing, gaskets, O-rings and other automotive components; for copier roll surface-covering materials; and for various sealing materials, etc. As described below, in the past, numerous compositions have been suggested for use in the forming of such silicone rubber sponge.
For example, silicone rubber sponge-forming compositions obtained by adding thermally decomposable organic foaming agents represented by azobisisobutyronitrile are taught in Japanese Examined Patent Application Publication No. (hereafter referred to JP Kokoku) Sho 44-461 and Japanese Unexamined Patent Application Publication No. (hereafter referred to JP Kokai) Hei 7-247436 and a silicone rubber sponge-forming composition obtained by adding balloons having an outer shell of thermoplastic plastic containing butane, isopropane, or other volatile hydrocarbons inside is proposed in JP Kokai No. Hei 5-209080. However, thermally decomposable organic foaming agents and their thermal decomposition products are not good for health. In addition, since the compositions make use of gases generated by the decomposition of thermally decomposable foaming agents on heating as foaming agents or utilize thermoexpandable balloons as foaming agents, they are not easily amenable to compression molding and molding under hermetically sealed conditions, and although they are suitable for making moldings of simple shapes, they are not suitable for applications involving moldings of complex shapes and their applications are limited.
A silicone rubber sponge-forming composition, which comprises an alkenyl-containing diorganopolysiloxane, an organohydrogenpolysiloxane, a small amount of water, and a platinum catalyst and is foamed by hydrogen gas generated by a reaction between the organohydrogenpolysiloxane and water during crosslinking via hydrosilation, is taught in JP Kokai Sho 54-135865, a silicone rubber sponge-forming composition, in which liquid alcohol is used instead of water, is taught in JP Kokai Hei 5-70692, and a silicone rubber sponge-forming composition, which is a hydrosilation reaction-crosslinkable composition comprising an alkenyl-containing diorganopolysiloxane, an organohydrogenpolysiloxane, and a platinum catalyst and is foamed by excess organohydrogenpolysiloxane, water, and monohydric or polyhydric alcohol, is taught in JP Kokai Hei 11-130963. Such hydrosilation-crosslinkable, i.e. addition-curable type silicone rubber sponge-forming compositions pose safety problems because they all utilize hydrogen gas generated during curing as a foaming agent. Moreover, depending on the amount of material injected into the cavity during compression molding and injection molding, cell structure may become uneven, part dimensions in the mold and as-molded part dimensions may end up different, resulting in poor dimensional accuracy. A silicone rubber sponge-forming composition, which comprises an organopolysiloxane, an emulsifying agent, water, and a thickening agent and is foamed by means of evaporation and expansion of water, is taught in JP Kokai Hei 7-122000 and a silicone rubber sponge-forming composition, which is a hydrosilation-crosslinkable, i.e. addition-curable type composition comprising an alkenyl-containing diorganopolysiloxane, an organohydrogenpolysiloxane, and a platinum catalyst and which contains water absorbent polymer particles, is taught in JP Kokoku 2002-114860. However, despite being suitable for compression molding applications and for imparting superior dimensional accuracy to molded articles, i.e. sponge moldings, these compositions lack sufficient cell fineness and cell uniformity.
Additionally, a method for fabricating silicone rubber sponge by curing a condensation-curable type silicone emulsion comprising a diorganopolysiloxane having both ends blocked by hydroxyl groups and an organotin catalyst by freezing and, after thawing, removing water therefrom, is taught in JP Kokai Sho 59-12832. This method, however, leads to poor productivity due to the energy and time requirements imposed by freezing and thawing, as well as to the poor heat resistance resulting from the use of the organotin catalyst. On the other hand, despite the existence of well-known hydrosilation-crosslinkable, i.e. addition-curable type silicone rubber emulsion compositions and methods for producing silicone rubber and coating films from such compositions, there are no known addition-curable type silicone rubber sponge emulsion compositions or methods for producing silicone rubber sponge from such emulsion compositions.